1. Field of the Invention
The invention relates to a method and an apparatus for motion compensation of SAR images by means of an attitude and heading reference system.
2. Description of the Prior Art
An aircraft-borne radar system with synthetic aperture (SAR) operated by Applicants operates in the L, C and X band. Such radar systems are employed for imaging the earth's surface along the flight path. The antenna is aligned perpendicularly to the flight direction slanting downwardly, as indicated schematically in FIG. 8. This gives a land map, the image dots of which represent the radar reflectivity of the objects on the ground.
Generally, when processing a radar image ideal flight conditions are presumed, i.e. the heading, position and forward speed are assumed to be constant. However, in practice this is not true because the aircraft is deflected from its nominal flight path by turbulences and also varies in its forward speed. Deviations from the altitude and laterally of the flight direction result in a variation of the slant range between the antenna and an illuminated target on the ground and therefore influence the phase history of a backscatter signal. In addition, the varying forward speed prevents an equidistant scanning of the illuminated terrain strip. The motion errors also impair the azimuth compression and lead to a loss of quality of the radar images processed, resulting in geometrical distortions, reduction of the resolution and a decrease in the contrast.
Various methods are known for motion compensation. In a motion compensation having a master/slave system two inertial navigation systems (INS) are employed, the slave system being mounted in the vicinity of the antenna and the master system usually in the aircraft nose. The slave system is employed for short-time stable measurements whilst long-time stable measurements are carried out by means of the master system; the two measurements are then linked via a so-called Kalman filter.
A disadvantage of a motion compensation by means of a master/slave INS system is that the slave system is simply constructed and, as already stated, can be employed only for short-time measurements. In the case of long-time measurements, sensor errors produce a drift in the position computation. The slave system must therefore be supported by a long-time stable master system which however generally is part of an aircraft navigation system and as explained above located in the nose of the aircraft.
As a result, there is usually a long lever arm between the INS system and an antenna phase centre and this arm must be compensated in the calculations. With a very long lever arm, extremely high demands are made of the angular resolution of the inertial sensor and they cannot be met by any INS system. The master INS system alone can therefore be employed for movement compensation only with limited accuracy; simultaneously, the calculation of the Kalman filters for supporting the slave INS system is very complicated and additionally requires a correspondingly complex software and hardware.
Using global positioning systems (GPS systems) for motion compensation permits an accurate determination of the position and speed. In particular, a differential GPS system is suitable for movement compensation. However, a disadvantage of motion compensation methods involving such a GPS system is that it is dependent on support by a ground station. This firstly makes the operating costs very high and secondly leads to restrictions in the choice of the area of use. GPS motion data are however not accurate enough without a ground station.
In an autofocus method for motion compensation radar raw data are evaluation and normally only used for estimating the forward speed of the carrier. The autofocus method requires however a very high computing expenditure and consequently in real time systems makes high demands of the hardware. In addition, the bandwidth and accuracy with the autofocus method is not very high and with relatively large motion errors the compensation of the speed error alone is no longer enough to produce good image quality.
With motion compensation by means of the so-called reflectivity displacement method, the azimuth spectrum of the radar raw data is evaluated; this makes it possible to determine the forward speed and a phase error. A motion compensation is then carried out with this information. In the reflectivity displacement method as well a high computing expenditure is required.
Consequently, in real time this method can be implemented only by means of computers running in parallel. Furthermore, the bandwidth is limited and the separation of the speed and phase information presents problems when relatively large disturbance movements are involved.